1. Field of the Invention
The present invention relates to hydraulic valves, and more particularly to unbalanced, pilot-operated poppet valves.
2. Description of the Related Art
A wide variety of machines have moveable components which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve assembly. A common hydraulic valve assembly comprises four electrohydraulic valves arranged in a Wheatstone bridge configuration with each valve in a different leg of the square bridge. One corner of the bridge is connected to a supply conduit, while the opposite corner of the bridge is connected to a tank return conduit. The other two corners are connected to the chambers of the hydraulic actuator. Opening two valves on opposite legs of the bridge sends fluid from the supply conduit to one actuator chamber and drains fluid from the other chamber. Selecting a particular pair of valves determines the direction in which the hydraulic actuator moves.
The valves often are electrohydraulic poppet valves (EHPV) in which the poppet is pilot-operated by a linear actuator, such as a solenoid. Applying electric current to an electromagnetic coil of the solenoid moves an armature that acts on a pilot valve element to open a pilot passage in a main valve poppet. Opening the pilot passage releases pressure in a control chamber, thereby allowing the main valve poppet to move away from a valve seat which motion creates a path between the inlet and outlet of the valve. The amount that the main valve poppet moves, and thus the degree that the valve opens, is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature, another valve component, or both is spring biased to close the valve when electric current is removed from the electromagnetic coil
The pressure differential across the valve affects the speed at which the main valve poppet opens and closes. That pressure differential changes with variation in the load applied to the machine component operated by the hydraulic fluid from the valve, as well as by variation of the supply fluid pressure. In the closed state, the pressure differential can affect the amount of force required to open the valve and to produce a desired fluid flow through the valve.
The adverse effects of varying differential pressure were previously addressed by providing a stem on the pilot valve element which extended into the pilot passage in the main poppet, as shown in U.S. Pat. No. 5,878,647. Pressure on both sides of the main poppet were applied to different surfaces of this stem which caused the pilot poppet to move in a manner that opened and closed passages between the valve ports and the control chamber for the main poppet thereby counteracting the effects of variation in the differential pressure. This stem commonly is referred to as a pressure balancing stem. Nevertheless a pressure balanced valve of this type had durability, leakage, and pilot force sensitivities as compared to an unbalanced valve.
Although use of an unbalanced valve avoided those sensitivities, that type of valve did not always provide the dynamic response required for a some hydraulic systems, such as those for earth excavators. Specifically the closing speed of an unbalanced valve when the electric current terminated often was not fast enough to properly control the associated hydraulic actuator.
Therefore, it is desirable to improve the closing speed of an unbalanced electrohydraulic poppet valve.